I'm looking for open-source text-to-speech (TTS) engines written in C++. Ideally with high-quality voices (see quality definition below), but also lower quality alternatives are okay as long as the source is freely available.
Does such an open source project exist?
By "high-quality" I mean "human sounding", "non-robotic" and with end results roughly on par with these two English language examples: Example 1, Example 2
I think you'd find Flite an excellent project. It is widely used, open source, and multi platform. Developed at CMU, it has more rigor than a random developer's project.
However, it is written entirely in C for portability to mobile objects, perhaps reducing its academic value to you, if that is what you are after (looking for C++)
If you are not interested in a C library, that same site has a link to Festival, a C++ library from University of Edinburgh that is much more robust. However, it is much less portable and you'll have to work at it to get it working in your environment I'm sure.